This page is intended to cover Sections 2 and 3 of Article I, with the exception of Impeachment, which we will cover next. If you cross check to the full document, be sure to note that the 17th amendment replaced state legislature with popular vote as the means of selecting Senators and added the 2nd method of making a mid term replacement.
The key points are that we have a bicameral legislation, (two houses), with the lower house serving 2 year terms, and the upper house 6 years. We will get in to further details as to how they interact when we get to Section 7.

The Legislature - Qualifications

HouseSenateSenate (cont)
Age2530
Yrs Citizen79
Present Resident In StateYesYes
Term2 yrs6 yrs1/3 elected each two years
Elected byCitizensCitizens Prior to 1913 was
chosen by state legislature
Number per stateBased on Population2
Mid-term ReplacementSpecial ElectionSpecial Election
PresiderSpeaker of HouseVice President Votes only as tie breaker
Presider Chosen ByVote of HouseNational Election

The Senate has a President pro tempore, elected by them, who fills in if Vice President is acting as President.